Metallurgy, Properties, and Uses of Ferrous Metals and Alloys - III

Metallurgy, Properties, and Uses of Ferrous Metals and Alloys III

The Alloy Steels and Cast Irons

Function of Alloy Elements in Steel

To increase hardenability.

To increase resistance to softening on tempering.

To increase resistance to corrosion.

To improve high temperature properties.

To increase resistance to abrasion.

To strengthen ferrite.

The alloying elements produce these changes in the characteristics of steel in various ways. An increase of hardenability, the decrease in the rate of transformation is associated w/ the rate of diffusion of carbon and the alloying elements. Any change in the rate of diffusion will influence the amount of softening on tempering, since the softening depends on the degree on precipitation and coalescence of the phases.

Designations for Steel

The Society of Automotive Engineers (SAE) and American Iron and Steel Institute (AISI) established a series of specifications covering some of the alloy steels, which are designated by numbers. One point of carbon is equal to 0.01 percent.

The first figure refers to the type of alloy

Carbon

Nickel

Nickel-chrome

Molybdenum

Chromium

Chromium-vanadium

Tungsten

Nickel-chromium-molybdenum

Silicomanganese

The next figure refers to the series w/in that alloy group.

Last two figures refers to the points of carbon in the steel.

Nickel

Nickel is one of the first alloy steels to be developed. It increased the strength and toughness of the steel. These steels which are very important, contains from 2 to 5 per cent nickel and from 0.10 to 0.50 per cent. Nickel contributes great strength and hardness, w/ high elastic limit and good ductility.

Steels containing from 12 to 21 per cent nickel and about 0.10 per cent carbon have a good strength and toughness, and extremely good resistance to corrosion

Stainless steel containing 36 per cent nickel is used for surveyors tapes and other instruments because of its low coefficient of expansion.

Nickel steels containing from 2 to 5 per cent nickel are used for machine parts. A structural steel containing 3.5 per cent nickel will have high elastic and endurance limits. Steels containing larger amounts of nickel are used for stainless steels, Invar, and others.

Chromium

It is used in steels as an alloying element to combine hardness obtained by quenching w/ high strength and high elastic limit.

The most common chrome steels contains from 0.50 to 2.0 per cent chromium and from 0.10 to 1.50 per cent carbon.

The special chrome steels of the stainless variety contain from 11 to 17 per cent chromium.

To develop the maximum hardness, chromium steels must de quenched from well above critical temperature; that is, from 1800 F 1900 F for machinery steel.

The tensile strength and yield point increase rapidly, and the ductility decreases, as the carbon content decreases.

Chrome steels , when quenched and tempered at about 1200 F, have about double the tensile strength, triple the yield point, and have the ductility to the corresponding annealed steel.

The greatest use of chromium steel is in the manufacture of high grade balls, rollers, and races for bearings; and armor-piercing projectiles and armor plates. It is also used for tools, dies, gears, safes, and vaults.

High chromium steels are used to resist corrosion, to impart greater wear-resistance, and to increase the creep strength for high-temperature service.

Manganese

Manganese steel usually contains from 11 to 14 per cent manganese and from 0.8 to 1.5 per cent carbon, and possesses a combination of extreme hardness and ductility, acquired by quenching the steel in water, from 1800 F.

Manganese steel in the quenched condition is austenitic; if it is cooled slowly the carbides are precipitated by the grain boundaries. The steel tends to become martensitic and correspondingly brittle.

The tensile strength of water-quenched steels containing more than 7 per cent manganese increases rapidly with increasing amount of manganese and reaches a maximum of about 140,000 lb/in. with 13 to 14 per cent manganese. The elongation of 7 per cent manganese steel is about 1.5 per cent in 8 in., and increases with larger amounts of manganese to a maximum elongation of about 50 per cent in 8 in with 13 to 14 per cent manganese.

The elastic limit of these manganese steels is very low in proportion to the tensile strength, being only about 35 to 40 per cent of the tensile strength. The principal use of manganese steel is in machinery parts subject to severe wear, as in crushing and grinding machinery. Also it is extensively used in railroad equipment for frogs, switches, crossings, and curve rails.

Tungsten

It is usually employed in conjunction with other elements in alloy steel. Steels containing 3 to 18 per cent tungsten and from 0.2 to 1.5 per cent carbon are used for dies and cutting tools.

Those steels with high tungsten content have the property of retaining their hardness when hot, after being quenched from a high temperature.

Steels containing 4 to 5 per cent tungsten and 0.5 to 0.7 per cent carbon display very high magnetic reluctance, which makes them valuable in the construction of permanent magnets.

The tensile strength and elastic limit of steel reach a minimum of 10 to 12 per cent tungsten; with more tungsten, these properties decrease.

Tungsten steel principally use for cutting tools, dies, valves, taps and permanent magnets.

Molybdenum

The action of molybdenum in tool steel is very similar to tungsten in altering the critical points, hardening power and physical properties, and a given percentage of molybdenum can be used to replace an even larger of tungsten.

Molybdenum is used in connection with chromium in producing chrome-molybdenum steel, w/c has very good strength properties, especially resistance to repeated stress.

The chrome-molybdenum steels contain about 0.20 per cent molybdenum. They increase machinability for the same hardness, and they also increase the depth of hardness penetration when the steel is quenched.

Vanadium

Vanadium in amounts of 0.20 per cent, produces a marked increase in tensile strength and elastic limit in low- and medium- carbon steels w/o a corresponding loss of ductility.

It has also a property of assisting in the degasification of steel in the molten state, preventing the occlusion of gases in castings.

The structure of carbon vanadium steels is finer than that of plain carbon steels.

Chrome vanadium steels usually contain about 0.5 to 1.5 per cent chromium, 0.15 to 0.30 per cent vanadium, and 0.15 to 1.10 per cent carbon. They have extremely good tensile strength, elastic limit, endurance limit and ductility. These steels are used extensively for machinery.

Casting, forgings, parts such as springs, shafting, gears, pins, and steering knuckles, and many drop-forged parts are made of chrome-vanadium steel.

Silicon

Silicon steels contain from 1 to 2 per cent silicon and 0.1 to 0.4 per cent carbon, and resemble nickel steels very closely. These steels have a high elastic limit as compared to ordinary carbon steel.

A steel containing 3 per cent silicon, with the smallest possible amounts of carbon and impurities, has been employed with great success for electrical machinery.

Steels containing 5 to 18 per cent chromium, 1 to 2 per cent silicon, and 0.10 to 1.1 per cent carbon are sometimes employed to resist corrosion.

A steel containing less than 1 per cent manganese, about 2 per cent silicon, and 0.4 to 0.6 per cent carbon is used extensively for springs because its elastic limit is very high.

Chrome-Nickel

It is one of the most important groups of alloy steel. These alloys carry from 0.30 to 2.0 per cent chromium, from 1.0 to 4.0 per cent nickel, and from 0.10 per cent carbon. When heat treated, they acquired greatly increased tensile strength, elastic limit and endurance limit, together with toughness and ductility.

The structure of alloys of this group is about the same as that of plain nickel steel, except for the presence of chromium carbide.

The principal uses of this steels are in the manufacture of gears, forged, axles, crankshafts, propeller shafts, connecting rods, and machine parts generally.

A very important steel in the chrome-nickel series contains from 16 to 19 per cent chromium, 7 to 10 per cent nickel, and less than 0.15 per cent carbon. This steel is austenitic at all temperatures. Its physical properties can be altered only by cold working. One of these steels is commonly called 18-8 stainless steel.

Selection and Properties of Alloy Steel

The selection of alloy steel for any given requirement depends primarily upon their characteristics and their prices. The latter factor is important because, under certain conditioned, it may be more economical to use more steel having lower physical properties than the smaller amount of a more expensive steel.

Lightness is an important factor, where the quenched and tempered alloy steel serves as a very useful purpose. Often several steels may satisfy the requirements, in w/c case the least expensive one is selected.

In selecting of alloy steel other factors must be taken into account such as follows:

Machinability

Sensitiveness to the heat treatment operation

Ability to harden a fair depth, w/c is especially important where large pieces are to be heat-treated.

One of the most desirable properties is uniformity of structure and of mechanical properties. Some alloy steels seem to have a narrower range of variability than others when heat-treated to give a desired set of properties. The selection of alloy steel is best suited to meet a given set of conditions is a matter requiring considerable experience and sound engineering judgment.

Alloy Cast Iron

Using the elements nickel, chromium, molybdenum, and vanadium, is cheaper than cast steel and in many cases may be better suited to the purpose. These elements are used either singly or in combination and they influence the structure and properties of the cast iron in much the same way as they affect steel.

Nickel acts like silicon to promote graphitization, and is also produces a finer graphitic structure.

Automobile cylinders, pistons, rings, brake drums, forging dies, valves and a great variety of other products are being made of alloy cast iron. This materials can also be heat-treated to improve its properties.